The present invention relates to a backlight driving device, a backlight driving method, and a liquid crystal display device that control driving of a backlight device including LED (Light Emission Diode) elements.
While a CCFL (Cold Cathode Fluorescent Lamp) type backlight using a fluorescent tube is mainstream as a backlight source for a liquid crystal panel, there has been a demand for the backlight to be free of mercury in consideration of the environment. For this reason, LEDs have recently been considered promising as a light source to supersede the CCFL. A method of using the primary colors of red LEDs, green LEDs, and blue LEDs individually and obtaining white light by optical synthetic additive color mixing, in particular, has been considered actively to be used for television purposes because the method facilitates attaining a color balance.
When LEDs are used as a light source of a backlight, luminous efficiencies of red LEDs, green LEDs, and blue LEDs are different, and therefore a current flowing through an LED of each color needs to be independent of the other colors. Since an LED used for each color is different in semiconductor composition, element voltage is different for each color, and thus power consumption is different for each color. Also, when LEDs are used as a light source of a backlight, it is not possible to drive each LED separately from a practical viewpoint of cost (see Japanese Patent Laid-open No. 2001-272938).
Therefore, when LEDs are used as a light source of a backlight, a method of cascading a certain number of LEDs (series connection) and driving the LEDs en bloc is used.
Specifically, a method is used in which a DC-to-DC converter power supply and a PWM (Pulse Width Modulation) control unit are provided for each cascaded group of LEDs as a load, for example each cascaded group of a predetermined number of LEDs for each color of red, green, and blue. Photosensors for red, green, and blue detect light quantities of the respective colors, and an amount of current flowing through each group of LEDs is subjected to PWM adjustment by feedback control on the basis of a result of the detection to thereby adjust chromaticity and brightness resulting from a synthesis of red, green, and blue.